Image capturing devices such as sensor pixel arrays convert incident light into electrical signals and use the electrical signals to reproduce an image. The quality of the reproduced image is determined in part by the precision of the pixels themselves and in part by the precision of readout circuitry coupled to the sensor pixel array. Column fixed pattern noise (CFPN) is a result of variations, i.e., mismatches, in the readout circuitry due to small variations during manufacturing, temperature of the circuitry or operating time. Some of the parameters which impact CFPN include offset voltage, feedback capacitance value, transistor threshold voltage, overlap capacitance value and bias current variations. As the CFPN increases, the quality of the reproduced image decreases.
CFPN problems are separate from any degradation which occurs based on imprecision within the pixels themselves. Conventional techniques attempt to correct image degradation by using the combined effect of CFPN and pixel imprecision. However, when the intensity of incident light is low, image degradation issues with respect to CFPN are often more significant than image degradation introduced by the pixels. Conventional techniques fail to account for the relative increase in CFPN induced degradation at low light levels.
Readout circuitry is often electrically connected to gain amplifiers in order to boost signals output by the sensor pixel array to provide higher contrast in the reproduced image. As the gain amplifiers increase the strength of the signals output by the sensor pixel array, the impact of CFPN is also increased. Conventional techniques fail to account for the different amounts of CFPN introduced at different gain conditions.